The present invention relates to microsphere and/or pore containing ceramic particles used as proppants and for other uses. The present invention further relates to methods to make microsphere and/or pore containing ceramic particles to be used as proppants, reinforcing fillers, and other applications, such as where a combination of light weight and strength are required.
For many ceramic articles including ceramic particles, it is desirable to increase the strength of the ceramic body while decreasing its specific gravity (density). A method commonly used to decrease the specific gravity of a ceramic particle is to introduce porosity into the body of the ceramic. The introduction of pores into a ceramic body, however, typically causes a decrease in strength of the resulting pore containing ceramic particle. This effect is due in large part to the creation of stress concentrations in the ceramic created by the presence of the pores. The pores function as flaws in the surface structure that decreases the overall strength of the ceramic particle. The strength of pore containing ceramic materials decreases exponentially with increasing porosity. However, theoretical studies claim that strength will not show an exponential decay if the pores have a spherical shape and are smaller in size (Evans, et al., “Some Effects of Cavities on the Fracture of Ceramics: II, Spherical Cavities,” JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Volume 62, Issue 1, January 1979, Pages 101-106 and Chihiro Kawai and Akira Yamakawa, “Effect of Porosity and Microstructure on the Strength of Si3N4: Designed Microstructure for High Strength, High Thermal Shock Resistance, and Facile Machining,” JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Volume 80, Issue 10, pages 2705-2708).
A variety of granular particles are widely used as propping agents to maintain permeability in oil and gas formations. Three grades of proppants are typically employed: sand, resin-coated sand, and ceramic proppants. Conventional proppants offered for sale exhibit exceptional crush strength but also extreme density. Typical density of ceramic proppants exceeds 100 pounds per cubic foot. Proppants are materials pumped into oil or gas wells at extreme pressure in a carrier solution (typically brine) during the hydrofracturing process. Once the pumping-induced pressure is removed, proppants “prop” open fractures in the rock formation and thus preclude the fracture from closing. As a result, the amount of formation surface area exposed to the well bore is increased, enhancing recovery rates. Proppants also add mechanical strength to the formation and thus help maintain flow rates over time. Proppants are principally used in gas wells, but do find applications in oil wells.
Relevant quality parameters include: particle density (low density is desirable), crush strength and hardness, particle size (value depends on formation type), particle size distribution (tight distributions are desirable), particle shape (spherical shape is desired), pore size (value depends on formation type and particle size, generally smaller is better), pore size distribution (tight distributions are desirable), surface smoothness, corrosion resistance, temperature stability, and hydrophilicity (hydro-neutral to phobic is desired). Lighter specific gravity proppants can be desirable, which are easier to transport in the fracturing fluid and therefore can be carried farther into the fracture before settling out and which can yield a wider propped fracture than higher specific gravity proppants.
Proppants used in the oil and gas industry are often sand and man-made ceramics. Sand is low cost and light weight, but low strength; man-made ceramics, mainly bauxite-based ceramics or mullite based ceramics are much stronger than sand, but heavier. Ceramic proppants dominate sand and resin-coated sand on the critical dimensions of crush strength and hardness. They offer some benefit in terms of maximum achievable particle size, corrosion and temperature capability. Extensive theoretical modeling and practical case experience suggest that conventional ceramic proppants offer compelling benefits relative to sand or resin-coated sand for most formations. Ceramic-driven flow rate and recovery improvements of 20% or more relative to conventional sand solutions are not uncommon.
Ceramic proppants were initially developed for use in deep wells (e.g., those deeper than 7,500 feet) where sand's crush strength is inadequate. In an attempt to expand their addressable market, ceramic proppant manufacturers have introduced products focused on wells of intermediate depth.
Resin-coated sands offer a number of advantages relative to conventional sand. First, resin coated sand exhibits higher crush strength than uncoated sand given that resin-coating disperses load stresses over a wider area. Second, resin-coated sands are “tacky” and thus exhibit reduced “proppant flow-back” relative to conventional sand proppants (e.g. the proppant stays in the formation better). Third, resin coating typically increases sphericity and roundness thereby reducing flow resistance through the proppant pack.
Ceramics are typically employed in wells of intermediate to deep depth. Shallow wells typically employ sand or no proppant. As will be described in later sections, shallow “water fracs” represent a potential market roughly equivalent to the current ceramic market in terms of ceramic market size.
The family of non-oxide based ceramic materials, specifically the carbides and nitrides of metallic materials, display exceptional mechanical, thermal and chemical properties all of which in combination would be ideal candidates for a proppant system. Although, they display very high intrinsic failure strength, hardnesses, and fracture toughnesses, their apparent properties are highly dependent upon the microstructure of the ceramic material that develops during the sintering stage. Significant research has been conducted in the sintering of the carbide and nitride class of materials, the most important of which is the use of a glass forming liquid phase sintering aid to assist with the densification of the system. When using materials such as silicon carbide, care must be taken to avoid oxidation of the silicon carbide. The production of silicon dioxide and either carbon monoxide or carbon dioxide weakens the resulting proppant. Although, the liquid phase sintering approach assists with the densification, the properties of such a system are less than optimal and fail to reach the intrinsic properties that these materials are capable of, due primarily to the effects of a relatively weak phase existing at the grain boundaries of the ceramic material. In addition, with the liquid phase sintering approach, a high level of shrinkage occurs during sintering. The shrinkage is dependent upon a number of parameters, the most critical of which is particle size. Typically the shrinkage can approach 20% or higher.
Another approach to improve the sintering and consequently the properties of such ceramic systems has been with a reaction mechanism that forms the appropriate carbide and/or nitride phase directly from the metallic phase. In this method, a preform of the appropriate metal is produced, with approximately 25-30% percent residual porosity. The component is then subjected to thermal treatments under the appropriate atmosphere to induce the formation of the carbide or nitride phase. During the formation of the carbide or nitride phase, a volume increase occurs, which serves to close the residual porosity and yield a highly dense ceramic body that is more or less pore free. By carefully controlling the initial porosity of the preform, the volume expansion associated with the formation of the carbide or nitride phase will completely fill all internal porosity placement and/or size and the outer volume of the preform will remain unchanged. This process is termed net shape forming.
While having porosity in proppants can have advantages with respect to lowering the density or specific gravity of the overall proppant, as stated above, the pores can contribute to a lower crush strength of the overall proppant. It would be advantageous to form pores that not only lower the overall density or specific gravity of the proppant, but also do not contribute to loss of strength (e.g., crush strength) of the overall proppant.